This invention relates to the art of calendering thermoplastic materials into sheet-like form. More particularly, the present invention relates to an improved method and apparatus for removing the hot thermoplastic material from the calender.
Calendering of thermoplastic materials into film or sheet-like form is well known, and may be briefly described in relation to the manufacture of unplasticized ("rigid") polyvinyl chloride sheet. The PVC formulation--which generally comprises PVC resin, an impact modifier, a thermal stabilizer, one or more lubricants for "internal" and "external" lubrication, and pigments as well as various other processing additives--is mixed to distribute the additives throughout the PVC resin. Mixing may be accomplished using either large bulk mixers such as ribbon blenders or high speed mixers. The blended PVC formulation is then fluxed into one of several possible forms including small strands, chunks, and continuous ribbons. Typically, the fused PVC formulation is then passed between at least one set of mixing rolls--normally operated at from about 170.degree. to about 180.degree. C.--which serves to convert the fluxed PVC formulation into a relatively narrow, thick strip of material which is then passed to the calender by a conveyor.
Calenders are well known to those in the art and may be described as a series of rotating cooperating rollers or bands between Which is forced a thermoplastic material which, due to the action of the calender, assumes the shape of a sheet, film or web of specified width and thickness. Rigid PVC calenders usually consist of four rolls, although both three and five roll calenders are known. The surface of the formed sheet, film or web may be smooth or matte, depending on the finish of the final two calender rolls. As indicated above, there is a great variety possible in the number and configuration of rolls in a calender train. Those of ordinary skill in the calender arts will be familar with 2-roll, 3-roll, 4-roll or 5-roll types, which can be arranged for example in 2-roll vertical, 2-roll inclined, 2-roll horizontal, 3-roll vertical, 3-roll 120.degree., 3-roll inverted "L", 3-roll triangular, 4-roll stack, 4-roll "L", 4-roll inverted "L", 4-roll flat "Z", 4-roll inclined "Z", 4-roll vertical "Z", or 5-roll "L" configurations. For the processing of thermoplastics, particularly rigid PVC, the most commonly used calenders are the 4 - or 5-roll "L" or inverted "L" types, or the 4-roll inclined "Z" type (also known as an "S" calender).
The calendered PVC sheet is typically removed from the last roll of the calender by two to four stripper rolls, passed over two to six cooling drums, scanned by a beta ray gauge measuring device, cut by an edge trimming system and finally wound or stacked.
Embossing units are sometimes used in the manufacture of rigid PVC sheet, especially matte-finish products. An embossing unit typically comprises a rubber nip roll running against a patterned or matte finish steel roll.
Rigid PVC calenders typically operate from about 180.degree. to 200.degree. C. The object is to process the PVC formulation as quickly as possible through the calender line before it decomposes or sticks to the calender rolls. Unfortunately, adhesion of hot PVC to the last calender roll at the point it is stripped off the roll remains a problem. Some of the design and processing variables which affect the degree of adhesion are calender roll temperature, roll surface, the PVC formulation, plastic draw, and the design of the stripping section. Unfortunately, minimizing film adhesion by lowering roll temperature, using gloss calender rolls, and employing high draw ratios between the calender and the stripping rolls may lead to a commercially unacceptable PVC product. For example, low roll temperatures increase haze in transparent films and may cause other processing problems. High draw ratios between the calender and the stripping roll may create machine direction shrinkage in the film and increase gauge profile variations. In short, the basic problem in typical calendering operations is to remove the hot calendered thermoplastic sheet from the last calender roll and cool it without disturbing its lay-flat, gauge profile, surface characteristics or shrinkage properties.
These properties, especially lay-flat or curl resistance, and low levels of shrinkage generally, have become increasingly important in certain applications of calendered plastic materials. For example, calendered polyvinyl chloride is used to manufacture floppy disk jackets and also in blister packaging. Floppy disk jacket applications have increasingly demanded PVC sheets which will not curl or wrinkle upon exposure to temperatures up to about 160.degree. F. Blister packaging thermoformers require a shrinkage resistant film which will not curl in machines which do not restrain the film edges during heating. PVC resin which has been calendered tends to stick to the last roll of the calender and must be pulled off the roll. This tends to impart machine direction thermal shrinkage in the PVC sheet, rendering the film unattractive for floppy disk jacket and blister packaging applications.
One solution to this problem is proposed by F. Nicoll, "Manufacture of Continuous Plastic Sheets," U.S. Pat. No. 4,311,658 (Jan. 19, 1982), which is expressly incorporated by reference in its entirety. Nicoll '658 discloses rotation of the stripper rolls in the same direction as the direction of rotation of the last calender roll in order to minimize problems associated with the release of the hot PVC resin from the last calender roll, including the problem of surface uniformity in matte sheet manufacture and the problem of residual strain in general. Nicoll '658 does not directly address the problems associated with an unstable "stripping line", however.
The tendency of the hot calendered PVC film to stick to the last calender roll can result in a non-linear and/or a fluctuating "stripping line," which is the line across the last calender roll formed by the last point of film contact. A stable, linear stripping line is critical to production of high quality calendered film. A curved stripping line generally imparts a tendency to curl to the calendered film. A fluctuating stripping line caused by sporadic sticking of the film generally imparts web shrinkage variability to the film. The present invention should substantiallY eliminate the need for production personnel to constantly monitor the "stripping line" of the calendered film to ensure that it is stable.